The semiconductor industry has seen a major shift from leads to balls and from wires to bumps. This requires infrastructure developments and promises new opportunities. Ball grid array (BGA) packages are increasingly found in products including personal computers, portable communications devices, workstations/servers, mid-range and high-end computers, network and telecommunications systems, and even automotive applications. More specifically, BGA packages are being used in high speed circuits such as processors, application specific integrated circuits (ASICs), storage controllers, video controllers, programmable logic devices (PLDs), field programmable gate array (FPGA), etc.
Today's BGA packages boast high clock speeds, fine pitch structures and high pin-counts. When these packages are assembled onto a PC board, they perform predetermined functions at certain speeds. BGA packages continue to evolve toward smaller overall package dimensions that have an increased number of leads. Moreover, system designers require these devices be capable of operating at high frequencies. Such devices may include a complex array of closely spaced electrical leads adapted for establishing electrical communication with a semiconductor die, each lead having one end electrically connected to the semiconductor die. An opposing end is generally adapted for electrical connection to an external device, e.g., a printed circuit board. A conventional BGA package includes a semiconductor die secured to a die-attach pad formed on an upper surface of a substrate. The BGA package also includes a plurality of electrical leads adapted to provide electrical communication between the semiconductor die and one or more external devices. The semiconductor die and at least a portion of each electrical lead may be encased by an encapsulant material or, alternatively, the conventional BGA package may have no encapsulant material, depending upon the particular package construction and intended use.
In BGA packages, each of the electrical leads includes an external ball lead configured for electrical connection to an external device. The ball lead may be secured to a conductive pad formed on a lower surface of the substrate. Typically, each electrical lead further comprises a conductive via extending from the conductive pad and through the substrate to a conductive trace. The conductive trace is formed on either the upper or lower surface of the substrate or can be formed on inner layers of the substrate. The conventional BGA package may include a plurality of the ball leads arranged, for example, in an array or arrays of mutually adjacent rows and columns. The arrangement of ball leads is typically referred to as the “pin-out” or the “footprint” of the BGA package.
Electrical modeling of the package structures is often used to ensure adequate electrical performance. In addition to modeling the electrical behavior of a device, it is often desirable to directly measure certain electrical characteristics using measuring instruments in order to validate the electrical model. The demand for higher performance digital (and analog) systems ultimately requires higher bandwidth components. Moreover, one of the most common components used in modern digital and mixed signal systems is the ball-grid-array (BGA) package. The BGA protects the integrated circuit (IC), offers a low thermal path between the IC and the ambient environment, and provides electrical connection from the densely spaced IC pads to the less densely spaced solder balls. The design of the BGA electrical connections is critical to meeting the high frequency requirements. Therefore it is important to accurately test these electrical connections.
The main obstacle with producing high accuracy tests is the lack of a very high bandwidth interface between the test equipment and BGA package. High accuracy tests usually require a signal and ground point at both the solderball and IC connection sides. Furthermore, most high frequency paths are differential meaning that every signal is comprised of a pair (true and complement) of connections.
Published techniques usually entail the use of a fixture board to which the BGA is soldered. Then high frequency paths are routed a short distance on the fixture board to enable probing with readily available medium-pitch microwave probes. Or, alternatively, vias are used on the fixture board and direct probe contacts are made to the backside of the fixture board. Connections between the test equipment to the IC connections on the package are much more difficult. Either they are not done at all or depend on direct probing with readily available fine-pitch microwave probes. Providing that the locations of ground and signals are compatible with the fixed position probe tips, this technique is possible, but the quality of the probe connection is usually poor.
It can be seen then that there is a need for an apparatus for performing high frequency electronic package testing.